1. Field of the Invention
The present invention relates to a noise detector and a noise detection method for detecting noise of an image signal as well as a signal processor and a signal processing method employing the noise detector and the noise detection method.
2. Description of the Prior Art
For example, Japanese Patent Laying-Open No. 7-307942 discloses a conventional signal processor eliminating noise from an image signal. FIG. 5 is a block diagram showing the structure of the conventional signal processor.
The signal processor shown in FIG. 5 comprises a vertical adaptive filter 8 and a horizontal adaptive filter 9. The vertical adaptive filter 8 includes a vertical logical filter 41, a vertical unevenness determination part 42 and a selector 43. The horizontal adaptive filter 9 includes a horizontal logical filter 44, a horizontal unevenness determination part 45 and a selector 46.
When an image signal is input, the vertical unevenness determination part 42 determines the magnitude of vertical unevenness around each pixel and outputs the result of the determination to the selector 43. When the vertical unevenness determination part 42 determines that the vertical unevenness around the pixel to be processed is large, the selector 43 outputs a signal of the pixel to the horizontal adaptive filter 9 as such. When the vertical unevenness determination part 42 determines that the vertical unevenness is small, the selector 43 outputs the image signal from which vertical noise is eliminated by the vertical logical filter 41 to the horizontal adaptive filter 9.
Then, the horizontal unevenness determination part 45 determines the magnitude of horizontal unevenness around each pixel and outputs the result of the determination to the selector 46. When the horizontal unevenness determination part 45 determines that the horizontal unevenness around the pixel to be processed is large, the selector 46 outputs the image signal output from the vertical adaptive filter 8 as such. When the horizontal unevenness determination part 45 determines that the horizontal unevenness is small, the selector 46 outputs the image signal from which horizontal noise is eliminated by the horizontal logical filter 44.
Through the aforementioned operations, the signal processor shown in FIG. 5 can eliminate noise from segments or edges included in the image signal without deteriorating the resolution, i.e., without blurring the segments or edges.
While the aforementioned conventional signal processor can reproduce large-amplitude edges without blurring the same, however, the vertical and horizontal logical filters 41 and 44 disadvantageously blur portions continuously exhibiting fine details, such as the surfaces of clothes, the skin of a human being and a wood, for example, specific to the image signal and reduce the texture thereof.
An object of the present invention is to provide a signal processor and a signal processing method capable of properly correcting an image signal without reducing the texture specific to the image signal also in a portion continuously exhibiting fine details.
Another object of the present invention is to provide a noise detector and a noise detection method, employed for the aforementioned signal processor and signal processing method, capable of detecting mosquito noise and ringing of an image signal.
A noise detector according to an aspect of the present invention comprises a high-frequency component extraction circuit extracting a high-frequency component of an input image signal, a level detection circuit detecting the level of the high-frequency component extracted by the high-frequency component extraction circuit and outputting the result of detection as a level signal and a noise detection circuit detecting a small-amplitude edge around a large-amplitude edge of the image signal as noise on the basis of the level signal detected by the level detection circuit.
The noise detector extracts the high-frequency component of the image signal and detects the level thereof, whereby the small-amplitude edge around the large-amplitude edge of the image signal can be detected on the basis of the detected level and this portion can be detected as noise. Consequently, mosquito noise and ringing caused around the large-amplitude edge of the image signal can be detected.
The noise detection circuit may include an extension circuit outputting an extended signal obtained by extending the peak of the level signal detected by the level detection circuit, a conversion circuit outputting a 1/n signal obtained by converting the level of the extended signal output from the extension circuit to 1/n times and a small-amplitude detection circuit comparing the 1/n signal output from the conversion circuit with the level signal output from the level detection circuit for detecting the small-amplitude edge around the large-amplitude edge of the image signal.
In this case, the small-amplitude edge, existing around the large-amplitude edge corresponding to the peak of the level signal, smaller than 1/n of the level of the large-amplitude edge can be detected by comparing the 1/n signal obtained by extending the peak of the level signal and compressing the same to 1/n times with the level signal, and the small-amplitude edge can be detected as mosquito noise and ringing.
The noise detection circuit may further include a large-amplitude detection circuit comparing the extended signal output from the extension circuit with a prescribed reference value for detecting a large-amplitude edge having a greater level than the reference value, for detecting the small-amplitude edge detected by the small-amplitude detection circuit as noise with respect to the large-amplitude edge detected by the large-amplitude detection circuit.
In this case, the large-amplitude edge, which is a signal specific to the image signal having a greater level than the reference value can be detected by comparing the extended signal obtained by extending the peak of the level signal corresponding to the large-amplitude edge with the prescribed reference value. Therefore, a small-amplitude edge around the large-amplitude edge, which is a signal specific to the image signal, can be detected as noise, and mosquito noise and ringing can be more correctly detected.
The small-amplitude detection circuit may output a first detection result signal indicating that the 1/n signal output from the conversion circuit is greater than the level signal output from the level detection circuit, the large-amplitude detection circuit may output a second detection result signal indicating that the extended signal output from the extension circuit is greater than the reference value, and the noise detection circuit may further include a logic circuit ANDing the first detection result signal output from the small-amplitude detection circuit and the second detection result signal output from the large-amplitude detection circuit.
In this case, the first detection result signal indicates a portion where the 1/n signal is greater than the level signal, i.e., a portion excluding a large-amplitude edge portion while the second detection result signal indicates a portion where the extended signal is greater than the reference value, i.e., a small-amplitude edge portion around the large-amplitude edge including the large-amplitude edge portion. Therefore, only the small-amplitude edge around the large-amplitude edge can be detected by ANDing the outputs. Consequently, only mosquito noise and ringing can be detected from the portion around the large-amplitude edge of the image signal with a simple structure employing a binary signal.
The high-frequency component extraction circuit may include a horizontal extraction circuit extracting a horizontal high-frequency component of the input image signal, a vertical extraction circuit extracting a vertical high-frequency component of the input image signal and an adder circuit adding the horizontal high-frequency component extracted by the horizontal extraction circuit and the vertical high-frequency component extracted by the vertical extraction circuit to each other, the level detection circuit may detect the level of the sum of the high-frequency components by the adder circuit and outputting the result of detection as the level signal, and the noise detection circuit may detect horizontal and vertical small-amplitude edges around horizontal and vertical large-amplitude edges of the image signal on the basis of the level signal detected by the level detection circuit.
In this case, the horizontal and vertical high-frequency components are extracted and added to each other, and the level of the sum of the high-frequency components is thereafter detected so that the horizontal and vertical small-amplitude edges around the horizontal and vertical large-amplitude edges of the image signal can be detected as noise on the basis of the level signal. Therefore, horizontal and vertical mosquito noise and ringing caused around the horizontal and vertical large-amplitude edges of the image signal can be detected. The extracted horizontal and vertical high-frequency components are added to each other for performing subsequent horizontal and vertical processing in common, whereby the structure following addition is so simplified that the horizontal and vertical mosquito noise and ringing can be detected with a simple structure.
A signal processor according to another aspect of the present invention comprises a detection circuit detecting horizontal and/or vertical small-amplitude edge(s) around horizontal and/or vertical large-amplitude edge(s) of an input image signal as noise and a processing circuit performing horizontal and/or vertical correction of the image signal in response to the result of detection in the detection circuit.
The signal processor detects the horizontal and/or vertical small amplitude edge(s) around the horizontal and/or vertical large-amplitude edge(s) of the image signal as noise thereby detecting mosquito noise and ringing of the image signal and performing horizontal and/or vertical correction of the input image signal in response to the result of detection. Therefore, the image signal can be properly corrected in response to the states of mosquito noise and ringing. Consequently, the image signal can be properly corrected without reducing the texture specific to the image signal also in a portion continuously exhibiting fine details.
The processing circuit may include a smoothing circuit performing horizontal and/or vertical smoothing of the image signal in response to noise detected by the detection circuit.
In this case, the image signal is subjected to horizontal and/or vertical smoothing in response to the detected noise, i.e., mosquito noise and ringing, whereby proper smoothing can be performed in response to the states of the mosquito noise and the ringing.
The smoothing circuit preferably performs horizontal and/or vertical smoothing of the image signal when the detection circuit detects noise.
In this case, the image signal is subjected to horizontal and/or vertical smoothing when noise, i.e., mosquito noise and ringing take place, whereby only mosquito noise and ringing can be smoothed for implementing an image having neither mosquito noise nor ringing.
The processing circuit may include a contour correction circuit performing horizontal and/or vertical contour correction of the image signal in response to noise detected by the detection circuit.
In this case, the image signal is subjected to horizontal and/or vertical contour correction in response to the detected noise, i.e., mosquito noise and ringing, whereby proper contour correction can be performed in response to the states of the mosquito noise and the ringing.
The contour correction circuit preferably performs the horizontal and/or vertical contour correction of the image signal when the detection circuit detects no noise.
In this case, the image signal is subjected to horizontal and/or vertical contour correction when no noise, i.e., neither mosquito noise nor ringing takes place, whereby a high-quality image enhanced in an edge portion specific to the image signal can be implemented without enhancing mosquito noise and ringing.
The detection circuit may include a high-frequency component extraction circuit extracting a high-frequency component of the input image signal, a level detection circuit detecting the level of the high-frequency component extracted by the high-frequency component extraction circuit and outputting the result of detection as a level signal and a noise detection circuit detecting the small-amplitude edge around the large-amplitude edge of the image signal on the basis of the level signal detected by the level detection circuit.
In this case, the high-frequency component of the image signal is extracted for detecting the level thereof, whereby the small-amplitude edge around the large-amplitude edge of the image signal can be detected on the basis of the detected level and this portion can be detected as noise. Therefore, mosquito noise and ringing caused around the large-amplitude edge of the image signal can be detected.
The noise detection circuit may include an extension circuit outputting an extended signal obtained by extending the peak of the level signal detected by the level detection circuit, a conversion circuit outputting a 1/n signal obtained by converting the level of the extended signal output from the extension circuit to 1/n times and a small-amplitude detection circuit comparing the 1/n signal output from the conversion circuit with the level signal output from the level detection circuit for detecting the small-amplitude edge around the large-amplitude edge of the image signal.
In this case, the small-amplitude edge, existing around the large-amplitude edge corresponding to the peak of the level signal, smaller than 1/n of the level of the large-amplitude edge can be detected by comparing the 1/n signal obtained by extending the peak of the level signal and compressing the same to 1/n times with the level signal, and this small-amplitude edge can be detected as mosquito noise and ringing.
The noise detection circuit may further include a large-amplitude detection circuit comparing the extended signal output from the extension circuit with a prescribed reference value for detecting a large-amplitude edge having a greater level than the reference value, for detecting the small-amplitude edge detected by the small-amplitude detection circuit as noise with respect to the large-amplitude edge detected by the large-amplitude detection circuit.
In this case, the large-amplitude edge, which is a signal specific to the image signal having a greater level than the reference value, can be detected by comparing the extended signal obtained by extending the peak of the level signal corresponding to the large-amplitude edge with the prescribed reference value. Therefore, a small-amplitude edge around the large-amplitude edge, which is a signal specific to the image signal, can be detected as noise, and mosquito noise and ringing can be more correctly detected.
The small-amplitude detection circuit may output a first detection result signal indicating that the 1/n signal output from the conversion circuit is greater than the level signal output from the level detection circuit, the large-amplitude detection circuit may output a second detection result signal indicating that the extended signal output from the extension circuit is greater than the reference value, and the noise detection circuit may further include a logic circuit ANDing the first detection result signal output from the small-amplitude detection circuit and the second detection result signal output from the large-amplitude detection circuit.
In this case, the first detection result signal indicates a portion where the 1/n signal is greater than the level signal, i.e., a portion excluding a large-amplitude edge portion while the second detection result signal indicates a portion where the extended signal is greater than the reference value, i.e., a small-amplitude edge portion around the large-amplitude edge including the large-amplitude edge portion. Therefore, only the small-amplitude edge around the large-amplitude edge can be detected by ANDing the outputs. Consequently, only mosquito noise and ringing can be detected from the portion around the large-amplitude edge of the image signal with a simple structure employing a binary signal.
The high-frequency component extraction circuit may include a horizontal extraction circuit extracting a horizontal high-frequency component of the input image signal, a vertical extraction circuit extracting a vertical high-frequency component of the input image signal and an adder circuit adding the horizontal high-frequency component extracted by the horizontal extraction circuit and the vertical high-frequency component extracted by the vertical extraction circuit to each other, the level detection circuit may detect the level of the sum of the high-frequency components obtained by the adder circuit and outputting the result of detection as the level signal, and the noise detection circuit may detect horizontal and vertical small-amplitude edges around horizontal and vertical large-amplitude edges of the image signal on the basis of the level signal detected by the level detection circuit.
In this case, the horizontal and vertical high-frequency components are extracted and added to each other, and the level of the sum of the high-frequency components is thereafter detected so that the horizontal and vertical small-amplitude edges around the horizontal and vertical large-amplitude edges of the image signal can be detected as noise on the basis of the level signal. Therefore, horizontal and vertical mosquito noise and ringing caused around the horizontal and vertical large-amplitude edges of the image signal can be detected. The extracted horizontal and vertical high-frequency components are added to each other for performing subsequent horizontal and vertical processing in common, whereby the structure following addition is so simplified that the horizontal and vertical mosquito noise and ringing can be detected with a simple structure.
A noise detection method according to still another aspect of the present invention comprises steps of extracting a high-frequency component of an input image signal, detecting the level of the extracted high-frequency component and outputting the result of detection as a level signal, and detecting a small-amplitude edge around a large-amplitude edge of the image signal as noise on the basis of the level signal.
The high-frequency component of the image signal is extracted for detecting the level thereof in the noise detection method, whereby the small-amplitude edge around the large-amplitude edge of the image signal can be detected on the basis of the detected level and this portion can be detected as noise. Consequently, mosquito noise and ringing caused around the large-amplitude edge of the image signal can be detected.
The noise detection step may include steps of outputting an extended signal obtained by extending the peak of the level signal, outputting a 1/n signal obtained by converting the level of the extended signal to 1/n times, and comparing the 1/n signal with the level signal and detecting the small-amplitude edge around the large-amplitude edge of the image signal.
In this case, the small-amplitude edge, existing around the large-amplitude edge corresponding to the peak of the level signal, smaller than 1/n of the level of the large-amplitude edge can be detected by comparing the 1/n signal obtained by extending the peak of the level signal and compressing the same to 1/n times with the level signal, and this small-amplitude edge can be detected as mosquito noise and ringing.
The noise detection step may further include a step of comparing the extended signal with a prescribed reference level and detecting a large-amplitude edge having a greater level than the reference value, for detecting the small-amplitude edge as noise with respect to the large-amplitude edge.
In this case, the large-amplitude edge, which is a signal specific to the image signal having a greater level than the reference value, can be detected by comparing the extended signal obtained by extending the peak of the level signal corresponding to the large-amplitude edge with the prescribed reference value. Therefore, a small-amplitude edge around the large-amplitude edge specific to the image signal can be detected as noise, and mosquito noise and ringing can be more correctly detected.
A signal processing method according to a further aspect of the present invention comprises steps of detecting horizontal and/or vertical small-amplitude edge(s) around horizontal and/or vertical large-amplitude edge(s) of an input image signal as noise and performing horizontal and/or vertical correction of the image signal in response to the result of detection of the noise.
In the signal processing method, mosquito noise and ringing of the image signal are detected by detecting the horizontal and/or vertical small-amplitude edge(s) around the horizontal and/vertical large-amplitude edge(s) of the image signal for performing horizontal and/or vertical correction of the input image signal in response to the result of detection. Therefore, the image signal can be properly corrected in response to the states of the mosquito noise and the ringing. Consequently, the image signal can be properly corrected without reducing the texture specific to the image signal also in a portion continuously exhibiting fine details.
The step of performing correction may include a step of performing horizontal and/or vertical smoothing of the image signal when noise is detected in the noise detection step.
In this case, the image signal is subjected to horizontal and/or vertical smoothing when noise, i.e., mosquito noise and ringing take place, whereby only the mosquito noise and the ringing can be smoothed for implementing an image having neither mosquito noise nor ringing.
The step of performing correction may include a step of performing horizontal and/or vertical contour correction of the image signal when no noise is detected in the noise detection step.
In this case, the image signal is subjected to horizontal and/or vertical contour correction when no noise, i.e., neither mosquito noise or ringing takes place, whereby a high-quality image having an enhanced edge portion specific to the image signal can be implemented without enhancing mosquito noise and ringing.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.